Diagnostic du cycle de l'eau et sources de contamination en contexte de forçage anthropique : cas du ruisseau Rousse, Québec

Abstract

The Ruisseau Rousse watershed (Oka, Québec) is a periurban agricultural territory where the intensification of anthropogenic pressures, water withdrawals, agricultural runoff, and industrial discharges, has progressively degraded water resource quality, threatening both their availability and the ecological integrity of its protected outlet, the Grande Baie within Parc national d'Oka. In a context further compounded by climate change, reconciling agricultural production with the preservation of aquatic biodiversity requires a fundamental rethinking of the interactions between human activities and the local water cycle. It is within this framework that the Lab-Eau-Rousse was established as a living laboratory, with the overarching goal of improving water availability and quality in agricultural settings through the identification of adaptive solutions for water cycle management. The present research project constitutes the first phase of this initiative, focused on the development of diagnostic and monitoring methods for the water cycle and water resource quality, through a multi-tracer approach combining stable water isotopes, major ions, dissolved organic matter fluorescence, and radon-222, deployed across six monitoring stations (SW1–SW6) from November 2023 to December 2024. Isotopic analyses, corroborated by electrical conductivity and major ion data, reveal contrasting hydrological periods: a winter period dominated by groundwater contributions, a spring period marked by snowmelt contributions to surface runoff, and a summer–autumn period characterized by increased surface runoff and precipitation influence. This hydrological framework governs all solute transport dynamics observed in the system. In winter, the dominance of groundwater baseflow homogenizes stream chemistry and concentrates nitrates at groundwater discharge zones. In spring, snowmelt activates a first-flush mechanism that generates the highest annual nitrate loads at the watershed scale. During the summer–autumn period, the increase in surface runoff mobilizes dissolved organic carbon from the upstream zone and amplifies diffuse nitrate inputs from the midstream reach, producing markedly greater spatial variability in contaminant concentrations. A radon-222 mass balance model spatially quantified groundwater inputs, with proportions reaching up to 62.6% during low-flow conditions in the upstream reach. Nitrate export is primarily driven by snowmelt through a first flush mechanism, while diffuse agricultural inputs dominate during spring and summer. The ministerial recommendation (CCME) of 3 mg-N/L is regularly exceeded, notably upstream where the maximum concentration reaches 7.05 mg-N/L in autumn. Dissolved organic matter is predominantly allochthonous, with the wetland zone adjacent to SW2 identified as the main source of dissolved organic carbon. The atypical hydroclimatic conditions of 2023–2024, reduced winter snowpack, exceptionally wet summer, and dry autumn, represent a partial analogue of projected climate trajectories for southern Québec and highlight the watershed's growing vulnerability to extreme events. These findings underscore the need for a thorough characterization of the watershed's wetland zones, the necessity of preserving them, and the importance of optimizing agricultural practices in the most vulnerable sectors.

Date	14 May 2026
Original language	French
Awarding Institution	École de technologie supérieure
Supervisor	Janie Masse-Dufresne (Supervisor) & Florent Barbecot (Co-supervisor)